ErbB2/Neu overexpression, which occurs in 20-30% of breast cancers, is associated with resistance to traditional chemotherapeutic treatment, making it essential that we identify novel therapeutic targets. To investigate molecular mechanisms of ErbB2-induced tumorigenesis, we performed gene expression profiling on tissues from the MMTV-Neu mouse model. In this analysis, the transcriptional regulator LMO4 was upregulated 5 fold. LMO4 induces hyperplasia and tumors in mice. In addition, LMO4 is upregulated in 56% of primary human breast cancers. Interestingly, our microarray data analysis constitutes the first time intrinsic induction of LMO4 has been implicated in a mouse model of breast cancer, making this model ideal to study cancers where LMO4 is overexpressed. Further studies in our laboratory suggest that LMO4 is involved in regulation of cell cycle proteins. Prompted by these data, we hypothesize that LMO4 is necessary for ErbB2 tumor properties via a mechanism that involves regulation of the cell cycle. To address this hypothesis, we propose to identify the mechanisms underlying transcriptional control of the LMO4 promoter by ErbB2 and determine if LMO4 is necessary for maintaining ErbB2-mediated transformation. [unreadable] [unreadable] [unreadable]